This invention relates to stablization systems for damping the rolling oscillations of ships in general and more particularly to improved apparatus for determining the phase difference between the roll oscillation of a ship and the oscillation of a liquid contained in a stablizing tank.
When damping a rolling motion of a ship using a stablizing tank the stablizing effect depends on the amount of liquid contained in the tank. In order to optimize a stablizing effect the tank content must be varied as a function of the frequency of the waves of sea motion striking the ship most frequently. This frequency depends on the depth of water, the direction and intensity of the wind, the distance from land, and the velocity and course of the ship.
There is an optimum stablizing effect when the phase difference between the motion of the liquid in the stabilizing tank and the ship's roll angle is .pi./2 [90.degree.]. In one previously developed stabilization device for a ship provisions are made to measure the oscillations of the tank liquid and the oscillations of the ship's roll and to generate signals which selectively actuate, through a control unit, devices for filling and emptying the tank until the phase of the liquid tank oscillation trails by approximately 90.degree.. Such is disclosed in German Auglegeschrift No. 1,919,601. However, means for directly measuring a ship's roll angle turn out to be quite costly in practice. It is much simpler to measure the ship's roll rate using means such as a gyro. The roll angle can then be determined from the measured roll rate through integration. However, in practice, in view of the very low frequencies, typically in the range from 0.02 to 0.2 Hz, of the sea motion, an integration cannot be carried out over a long periods of time because of drift in the real sensors and integrators which will unduly falsify the result. Only ac voltages up to a finite lower frequency limit which is determined by the quality of the integrating amplifier used and the capacity of the integrating capacitor can be intergrated with adequate accuracy. However, for an economical solution the lower frequency of real integrators is so high that at the normal frequencies of sea motion which occur the error is too great.
In view of these problems the need for an improved system for determining correctly the phase difference between the roll oscillation of a ship and the oscillation of a liquid contained in a stabilizing tank without drift of the sensors and amplifiers or integrators being used unduly falsifying the result of the measurements at the very low frequencies of sea motion becomes evident.